Is the sender's address the only way to identify the seller?
I don't see how this would work for a large websites that accept bitcoins. Since all of the transactions are visible, anyone can claim address x is theirs and that they sent the payment.
I guess the only way would be to generate a new address for each transaction? Seems messy and hard to keep track of things.
Also, I though I'd add another question since I mentioned large websites. Would it be possible to automatically notify the server when a payment arrives? I'm talking about something like paypal's IPN system. So that everything would be automated. If the person buys a subscription for example, then upon payment confirmation it gets activated automatically.